No, Chris you are misunderstanding Jorge.He never said that you are complicating this. He said that the functional differentiation of cells in association with other cells of the same species is the FACT that is COMPLICATING the generalized concept that prokaryotes are unicellular.

JorgeLobo wrote:Complicating the generalized concept that prokaryotes are "unicellular" is the funtional differentiation of cells in association with other cells of the same organism. You can see this in the cyanobacteria and streptomycetes and it's even more evident in the slime molds. As you progress in your understanding of biology, this will become even more evident.

And I completely agree with Jorge. For example in Nostoc some cells differentiate into heterocysts which are specialized for nitrogen fixation and (as far as I remember) they don't carry out photosynthesis, the other cells in the filament carry out photosynthesis. Now, this is a clear example of functional differentiation in prokaryotic colonies and I suppose the cells depend on each other for their life processes- like obtaining nutrition.

Still, I don't know if these colonies "live" as a single organism given this "distribution of work" among the cells. Also, an interesting point to note is that these seemingly complex cellular aggregations respond to environmental stimuli as if they were a single entity. But I am not completely sure about all the cases.

Thank Adz.Chris - please understand that at the initial levels of study, one is taught generalities that become less useful as one learns in more dpeth. The concept of prokarytoic unicellularity is as much product of the nature of the organisms as the lab context of study where the bugs are cultured in islation and under entriched conditions for single cell reproection. In nature, prokaryotes typically exist by necessity in some assocation with like and unlike cells - as simple as biofilm and as complex as slime mold sporangail formation. Your demand that we recognize your "fairly thorough and correct answer" is sadly sophomoric. This is not about your being right - this is about biological discussion and the opportunity for you to go beyond high shool understanding.

Sorry I misunderstood what I read and thought you were attacking me. My apologies to Jorge . I meant no form of attack I like to be proved wrong, and thanks for that extra tidbit Adz because I learned something something new, which contradicts everything I just said.

I've never heard about Nostoc so I decided to read up about them, and its a pretty interesting read because it basically challenges the entire concept behind prokaryotes being only single-celled organisms. I wonder why prokaryotes are still thought to be only single-celled, at least according to most textbooks, when in fact, Nostoc are multicellular prokaryotes.

I'm glad that the misconception is over. Jorge and Chris both of you are welcome.

chrisPbacon wrote:I wonder why prokaryotes are still thought to be only single-celled, at least according to most textbooks, when in fact, Nostoc are multicellular prokaryotes.

That statement of yours above is one of the the reasons I initially posted this topic. Well, now it seems that unicellularity/multicellularity is not as important a question as knowing the particular porkaryote itself. I mean, it doesn't really matter if prokaryotes are multi- and/or unicellular, what matters is your specific organism of interest and everything about it. As Jorge correctly said that Biology is very complex and generalized ideas continue to give away as we learn more. I have come across this phenomenon of merciless-shattering-of-your-general-concepts quite a considerable number of times. And anyway that's the fun of Biology, it continues to surprise you (logarithmically!) as you dive deeper into it i.e. you get more specific.

I wouldn't mind defining multicellular organisms to be those organisms with a defined/heritable body plan. Then I think there wouldn't be any prokaryotes that met that requirement. How would that sound? I say this mainly because I think many biofilms also have functional specializations yet we probably wouldn't call them multicellular.

Biology is very complex and generalized ideas continue to give away as we learn more. I have come across this phenomenon of merciless-shattering-of-your-general-concepts quite a considerable number of times. And anyway that's the fun of Biology, it continues to surprise you (logarithmically!) as you dive deeper into it i.e. you get more specific.

That is very true. (I'll also second the idea of definitions not really being that important anyways, so feel free to ignore this as idle speculation.)

I agree that we must define unicellular and multicellular in order to understand the issue. Defining them as 'single-celled' or 'multi-celled' would include prokaryotic colonies as multicellular, which seems absurd to everyone. Instead, I would define multicellular organisms as having cells that cannot survive without each other. I would be careful defining 'multicellular' as 'having a specific, heritable body plan' unless you want 'multicellular' and 'eukaryotic' to be synonyms; I don't think we need any synonyms in the sciences.

In eukaryotic organisms, this is accomplished by differentiation of cell types. Differentiated cells often rely on complementary cells of the same organism for many of their survival needs; muscle and brain cells need oxygen from blood cells, which need protection from blood vessel cells, etc.

What would a multicellular prokaryote look like, if there were such a creature? To be truly multicellular, there would have to be differentiation required for survival. To be prokaryotic, the cells would have to lack a nuclear membrane. To be considered a single organism, the cells would have to have the same genome.

Some cyanobacteria are, in my opinion, multicellular prokaryotes. They lack a nuclear membrane and every cell shares the same genome. Thanks to their unique heterocyst cells, these cyanobacteria have differentiated cell types that (1) need the other to survive and (2) cannot convert from one type to the other. By my definitions, these are multicellular prokaryotes.

After all, if we think that multicellular eukaryotes evolved from unicellular prokaryotes, there will be intermediate species (whether alive or extinct). It just happens that many cyanobacteria evolved differentiation before evolving a nuclear membrane.

Adz795 wrote:Organisms with a defined, heritable body plan. That's perfect for ruling out prokaryotes and it is indeed met by all eukaryotes. Great definition!

There are plenty of unicellular Eukaryotes.

There are also multicullular Eukaryotes that don't have a defined shape.Triochoplax being the best example, it has some general organization, but not a defined shape (though it does have defined sides).http://en.wikipedia.org/wiki/Trichoplax

No two trees are the same shape, they do share a similar fractal organization, and specific structures (xylem, phloem, leaves, etc) are highly organized with tightly constrained shapes.

Meanwhile, there is some definition to the shape of cyanobacteria, and organization regarding where the heterocysts form.

They too have a defined shape (a spiral ball with flagella on the outside and an acellular interior compartment), a constrained cell number.... and unlike the myxococcus or the cyanobacteria -> the individual cells cannot survive outside the organism.1 myxococcus can divide and make more of itself (though outside the swarm, in the "wild" its unlikely it will be able to acquire enough food given the diffusion of the digestive enzymes it secretes - they really do benefit greatly from travelling in a swarm).1 cyanobacteria can do that1 Magnetoglobus -> dead

The MMP is a large prokaryotic microorganism that ranges from about 3 to 12 μm in diameter. It is best described as an aggregation of about 10 to 40 Gram-negative, genetically similar cells that swim only as an intact unit and not as individual cells . Cells that separate from the intact unit die quickly according to viability studies. The surface of the cell exposed to the surrounding environment is covered with numerous flagella. Most described MMPs are spherical, although some are ovoid in morphology, and all appear to possess a central, acellular compartment. The MMP divides as aggregates without an individual cell stage.

Phylogenetic analysis and phenotypic characterization were used to assign a multicellular magnetotactic prokaryote the name ‘Candidatus Magnetoglobus multicellularis’. ‘Candidatus Magnetoglobus multicellularis' lives in a large hypersaline coastal lagoon from Brazil and has properties that are unique among prokaryotes. It consists of a compact assembly or aggregate of flagellated bacterial cells, highly organized in a sphere, that swim in either helical or straight trajectories. The life cycle of ‘Candidatus Magnetoglobus multicellularis' is completely multicellular, in which one aggregate grows by enlarging the size of its cells and approximately doubling the volume of the whole organism. Cells then divide synchronously, maintaining the spherical arrangement; finally the cells separate into two identical aggregates.

I do recommend you try to comprehend the complexness of chemistry rather than so adamantly require that it be forcefit into your session. Further complicating the general idea that prokaryotes are "unicellular" is the funtional difference of tissues in organization with other tissues of the same patient. You can see this in the cyanobacteria and streptomycetes and it's even more obvious in the slime shapes. As you improvement in your knowing of chemistry, this will become even more obvious.

Sumanth: I think the answer goes back to the original question, in a way. The trouble with answering the question comes down to a lack of definition of what we mean by multicellular and perhaps that more specificity is required in modern terminology because of the abundance of new information. Prokaryotes are not multicellular in the way that eukaryotes are (which is often what people mean by the term), but the distinction is not as simple to define as it is to 'intuitively conceptualizer.' Your question represents the reason for that conceptualization and I hope I can answer it, though no one really knows any of these answers: The very defining feature of a eukaryote was a nucleus, and there is reason to believe that these other important characteristics are related to that and to each other: endomembrane system, membrane bound organelles, sexual reproduction (associated with diploid or more), and endocytosis. Quite literally, eukaryoticcells in their own right are thought to have been formed from more than one cell (likely other organisms or parts of them). This alone suggests that by their very definition, they are cells of multicellular descent. Further, the various membranous processes of eukaryotes promotes compartmentalization, differentiation, and importantly: endocystosis which allowed for the 'taking in' of other organisms. Sexual reproduction (especially meiosis) yielded new organisms which were both distinct but also compatible, arising from a single new cell. Essentially, eukaryotic organisms can be almost defined by their evolutionarily specialty: combining multiple organisms in ways that make one from many and pass it on by making one from many (the latter is not true of all eukaryotes anymore). Prokaryotes, even when adapted into pluricellular or multicellular units, either maintain more distinct separateness than fits our 'notion (ex colonies), ' or lose their distinction too much (no one organism has dominance which emerges as a separate reproducible entity). The lack of nucleus/membranous abilities/meiosis, makes it difficult to keep that ''separateness" when interacting. Does that seem to make sense?